Thermally Integrated Process and Apparatus for Purification and Separation of Components of a Synthesis Gas

ABSTRACT

Process for the purification and separation of a synthesis gas stream containing hydrogen, carbon monoxide and carbon dioxide in which synthesis gas is purified in a purification unit (W) involving a methanol washing step to remove carbon dioxide, the carbon dioxide depleted synthesis gas is purified by adsorption (P) to produce a purified synthesis gas stream and the purified gas stream is sent to a cryogenic separation unit (C) where it is cooled and separated by cryogenic separation in a column of a column system, at least one stream ( 13 ) enriched in carbon monoxide is removed from a column of the column system, warmed and divided in two, one part of the stream being removed from the cryogenic separation unit as a first stream, the other part of the stream forming a second stream ( 13 B,  15 B) and being sent to the purification unit wherein it is warmed and the warmed second stream is mixed with the first stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT ApplicationPCT/EP2012/072522, filed Nov. 13, 2012, which claims the benefit ofEP11306476.0, filed Nov. 14, 2011, both of which are herein incorporatedby reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention refers to a thermally integrated process andapparatus for the purification and separation of components of asynthesis gas.

BACKGROUND

The term “synthesis gas” is generally applied to gaseous mixtures whichcan be used to synthesize other organic compounds. In this particularcase, it refers to a mixture of carbon monoxide, hydrogen, carbondioxide as main components and methane, water, nitrogen, acid gas (CO₂),sulphides (H₂S, COS), cyanides (HCN), ammonia and heavy hydrocarbons assecondary components.

Coal (or petcoke or residue) gasification is often used to producesynthesis gas, presents many advantages with regards to operating costsand is developing more and more, especially in countries like China.

Acid gases, such as carbon dioxide and hydrogen sulphide contained inthe synthesis gas are removed through a purification unit via washing bya solvent. As an example, the Rectisol® unit (RU) is a well knownphysical wash process to remove acid gases and sulphides in coldmethanol (−20° C.; −70° C.). In this process the synthesis gas is firstcooled against cold products produced by the purification unit itself,methanol being primarily injected into the synthesis gas to prevent theformation of ice. The synthesis gas is then sent to the bottom of awashing column where hydrogen sulphide and COS are removed by washingwith liquid methanol solvent rich in carbon dioxide, coming from theupper part of the column. The carbon dioxide contained in the synthesisgas is absorbed by washing with clean liquid methanol at the top of thecolumn. Purified synthesis gas, which is free from carbon dioxide andCOS and hydrogen sulphide, is withdrawn at the top of the column(absorber). The methanol rich in carbon dioxide and sulphur compounds isremoved at the bottom of the wash column and regenerated by a series ofexpansion steps down to a medium pressure, so as to produce a flash gas.The methanol rich in carbon dioxide from the upper section is alsoregenerated by expansion down to a lower pressure to produce a secondflash gas. The medium and low pressure expansions serve to remove theacid gases from the solvent. The solvent is then further regenerated bywarm regeneration so as to produce clean methanol.

The purified gas leaving the top of the absorber is then furtherpurified by adsorption and sent to a cold box where it may be treated bywashing with liquid nitrogen, liquid carbon monoxide or liquid methaneor by partial condensation.

Flash gas from the cold box may be produced from a phase separator of apartial condensation step or from an intermediate distillation column.

“Low Temperature Technology in Coal Refining” by W. H. Scholz, ChemicalEconomy and Engineering Review, Vol 14, January 1982 suggests that anammonia synthesis gas could be warmed in a Rectisol® plant.

SUMMARY OF THE INVENTION

One aim of the present invention is to optimize the heat balance(integration of cold) between the purification unit and the cold box.

According on an object of the invention, there is provided a process forthe purification and separation of a synthesis gas stream containinghydrogen, carbon monoxide and carbon dioxide in which synthesis gas ispurified in a purification unit involving a methanol washing step toremove at least carbon dioxide, the carbon dioxide depleted synthesisgas is purified by adsorption to produce a purified synthesis gas streamand the purified gas stream is sent to a cryogenic unit where it iscooled and separated by cryogenic separation in a column system toproduce at least one stream enriched in carbon monoxide, at least onesaid stream enriched in carbon monoxide is divided in two, one part ofthe stream being removed from a column of the cryogenic separation unitas a first stream, the other part of the stream forming a second streamand being sent to the purification unit wherein it is warmed and thewarmed second stream is mixed with the first stream.

Optionally:

-   -   the second stream is removed from the cryogenic separation unit        at a temperature below 20° C.    -   the second stream is removed from the cryogenic separation unit        at a temperature above −65° C.    -   the second stream is warmed in the purification unit to a        temperature between −40° C. and 40° C. and then mixed with the        first stream within the cryogenic separation unit.    -   the second stream is warmed against the synthesis gas upstream        of the methanol wash column scrubber and/or against an        intermediate liquid side-stream or bottom liquid stream from the        methanol wash column and/or against carbon dioxide depleted        synthesis gas from the methanol wash column and/or against a        stream removed from a flash column of the purification unit.

According to a further object of the invention, there is provided anapparatus for the purification and separation of a synthesis gas streamcontaining hydrogen, carbon monoxide and carbon dioxide including apurification unit comprising a methanol washing column and at least oneheat exchanger, wherein synthesis gas is purified to remove carbondioxide, an adsorption unit wherein the carbon dioxide depletedsynthesis gas from the purification unit is further purified byadsorption to produce a purified synthesis gas stream, a cryogenicseparation unit wherein the purified synthesis gas stream from theadsorption unit is cooled and separated by cryogenic separation in acolumn of a column system to produce at least one stream enriched incarbon monoxide, means for removing said at least one stream enriched incarbon monoxide from a column of the column system, a heat exchange linefor warming the stream, means for dividing the stream into a firststream and a second stream, means for removing the first stream from thecryogenic separation unit, means for removing the second stream from thecryogenic separation unit, means for sending the second stream to theheat exchanger of the purification unit and means for removing thewarmed second stream from the heat exchanger and mixing it with thefirst stream.

Optionally:

-   -   the means for removing the second stream from the cryogenic        separation unit are connected to the cryogenic separation unit        in such a way that the second stream is sent to the heat        exchanger of the purification unit at a temperature between        −65° C. and 20° C.    -   the apparatus comprises means for sending to the heat exchanger        synthesis gas upstream of the methanol wash column of the        purification unit and/or an intermediate liquid side-stream        and/or the bottom liquid stream from the methanol wash column        and/or carbon dioxide depleted synthesis gas from the methanol        wash column.    -   the purification unit includes a flash column and means for        sending liquid from the methanol wash column to the flash column        and means for sending a stream removed from the flash column to        the heat exchanger of the purification unit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of theinvention and are therefore not to be considered limiting of theinvention's scope as it can admit to other equally effectiveembodiments.

FIG. 1 shows an embodiment of the invention.

FIG. 2 shows an embodiment of the invention.

DETAILED DESCRIPTION

The invention will be described in greater detail with reference to theattached figures. FIG. 1 shows the process according to an embodiment ofthe invention and FIG. 2 shows the detail of a Rectisol® typepurification unit to be used in the process according to the invention.

In FIG. 1, one or several products from the cold box (CO, H₂, H₂+COmixture, flash gas and/or cold box purges) are heated up to ambienttemperature in an exchanger of the purification unit. The heated streamor streams is/are mixed with a corresponding stream or streams (havingthe same composition) at the cold box outlet or in cold box warmexchanger depending on the return temperature.

Synthesis gas 1 from a synthesis gas generation unit (not shown—gasifieras an example) is cooled in a heat exchanger 3 and sent to apurification unit W which uses a solvent washing step (methanol as anexample). The synthesis gas is treated in unit W to remove acid gases,such as carbon dioxide and hydrogen sulphide. The purified synthesis gas7 at −65° C. to −35° C. is sent to an adsorption unit P, to removesolvent and carbon dioxide traces, forming gas 11. Gas 11 at between−35° C. and −65° C. is divided into two parts. One part 11 is used tocool the synthesis gas in exchanger 3. The synthesis gas 11 is therebywarmed to a temperature around ambient, i.e. between 0° C. and 45° C.and is then sent to the cryogenic distillation unit C where it is cooleddown from the ambient temperature in warm heat exchanger HX and coldheat exchanger CX before being separated by cryogenic distillation in acolumn system to form a product gas 13, such as carbon monoxide and afurther gas 15 which is a flash gas or a purge gas, each gas beingremoved from a column of the column system.

The rest of gas 11 from adsorption unit P forms parts 11A which iswarmed up in the purification unit W to form warmed gas 11A which ismixed with warmed gas 11 from exchanger 3 and is sent at ambienttemperature to the cryogenic distillation. Gas 13A is formed by aportion of the product gas 13 of the cryogenic distillation unit butremoved from the warm heat exchanger HX at between −65° C. to 20° C. Gas13A is warmed in heat exchanger 3 against the feed synthesis 1 Gas 13Bis a portion of gas 13B, warmed in the purification unit and mixed withGas 13A; The mixture is sent to the warm heat exchanger HX or downstreamof the heat exchanger HX outside the cryogenic unit C.

Similarly a portion 15A of gas 15 is removed from the warm heatexchanger HX and/or the cold heat exchanger CX at between −65° C. to 20°C. and divided in two, part 15B being warmed in the purification unit Wand part 15A being warmed in exchanger 3. The rest of stream 15 iswarmed in heat exchanger HX to form a purge gas. The two warmed streamsare mixed to form stream 15A and sent back to stream 15. The mixingpoint is either within the warm heat exchanger HX via stream 15C ordownstream of the heat exchanger HX outside the cryogenic unit C.

FIG. 2 illustrates a Rectisol® process for acid gas removal withindication of locations for warming cold gases 11, 11A, 13A, 13B, 15Aand 15B (mentioned as gas A in FIG. 3). The process removes carbondioxide and hydrogen sulphide, in particular. The solvent used ischilled methanol with the advantages of ready availability, highstability and high solubility characteristics for CO₂, H₂ 5 and COSmeaning that the solvent circulation rate is relatively small comparedto other wash systems. This results in low utility consumption figures(steam, cooling water, electricity . . . ). The Rectisol® process isespecially suitable for high sour gas concentrations and high pressures.The apparatus includes a methanol absorption column K01 for removing CO₂and/or H₂S, a regeneration column K02 also called MP flash column, alow-pressure flash regeneration column K03 and a hot regenerator columnK04.

The raw synthesis gas 101 is cooled in a series of heat exchangers 103,105, 113. In the first heat exchanger 103, the raw feed gas is cooledagainst cold synthesis gas A coming from the cryogenic unit C, forexample a liquid nitrogen wash unit, and against the purified gas 112exiting the H₂S/CO₂ absorption column K01. In the second heat exchanger105, the raw feed gas is cooled against colder fluid A from thecryogenic unit. The condensed water and BTX/naphtha are then separatedin a gas separator 107 and sent to battery limits. Water is removed fromthe feed gas in order to avoid water build-up in the methanol loop: ifthe water content is too high, solubility of methanol drops. A waterwash column 109 is included to remove NH3. The gas 111 is sent to athird cooling step in exchanger 113. In the exchanger 113, the gas 111is then further cooled against gas A from the cryogenic unit C, againstthe purified gas 112 exiting the H₂S/CO₂ absorption column K01 andagainst CO₂ product 141 coming from the re-absorber CO₂ product columnK03. Additionally the gas may be cooled against recycle top gas 127coming from the MP flash column K02. The cooled gas 111 is then sent tothe H₂S/CO₂ absorption column K01.

The gas is sent to the bottom pre-wash section of the absorption columnK01 where trace components like NH₃, and HCN and BTX are absorbed with asmall stream of CO₂-sub-cooled-laden methanol. The pre-wash methanol 115leaves the bottom of the absorption column K01 and is sent to the hotregenerator column K04.

The synthesis gas is then routed up via a chimney tray into theH₂S-absorption section of the absorption column K01 (bottom section)where H₂S and COS are removed by dissolution in CO₂-saturated methanolcoming from the CO₂-absorbtion section (top section). The methanolsolvent is fed to the top of the Absorption section on flow controlproportional to the inlet gas flow. The CO₂+H₂S-laden methanol 116leaves the main sump of the absorber column K01 and is routed to theMP-Flash Column K02 for regeneration. The de-sulfurised gas then entersthe lower washing part of the CO₂-absorption section via another chimneytray. An additional sulphur-free feed gas stream may also be fed to thecolumn at this point.

In the CO₂-absorption section, the gas is washed with cold, flashregenerated methanol used as main wash methanol and with cold, hotregenerated methanol used as fine wash methanol. Due to the heat ofabsorption of the CO₂, the methanol 117, 123 is heated up considerablyand needs to be cooled on its way down the column. Therefore, in thelower part of the CO₂-absorbtion section, the gas is scrubbed withCO₂-laden methanol which is cooled against gas A from the cryogenic unitC in exchangers 119, 121. The cold purified gas 112 exiting the absorberK01 is then routed to the adsorption unit P of FIGS. 1 and 2.

The bottom liquid 135 from flash column K02 is sent to the top of columnK03 following cooling in exchanger 136 against stream A.

The LP flash column K03 receives the bottom liquid 135 and 137 fromcolumn K02 and produces a CO₂ rich stream 141 and an off-gas stream,which is sent to the atmosphere. In hot regenerator K04 the methanol isfinally regenerated by stripping with methanol vapours. The overheadsbeing an acid gas with sulphur content suitable to be processed in aClaus sulphur recovery unit. The bottoms product, stream 125, is thefinewash methanol fed on top of absorber K01.

The stream A is thus used to cool at least one of the following fluids:methanol removed at an intermediate point of the absorption column K01,synthesis gas which is to be treated in absorption column K01, bottomliquid from the MP flash column K02, which then feeds the LP flashcolumn K03.

Among the advantages of the invention are:

-   -   reduction of the adsorbent volume, of the drying bottle size, of        the bed regeneration flowrate and of the regeneration heater        steam consumption, as impurity adsorption is improved at low        temperature;    -   reduction of the cold box warm exchanger size;    -   optimization of the purification unit heat balance, the unit        operating at a lower temperature, thanks to the additional        refrigeration available at the cold box outlet; and    -   optimization of the overall purification unit efficiency and        cold box energy consumption (power consumption of frigorific        cycle compressor).

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing (i.e.,anything else may be additionally included and remain within the scopeof “comprising”). “Comprising” as used herein may be replaced by themore limited transitional terms “consisting essentially of” and“consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary arange is expressed, it is to be understood that another embodiment isfrom the one.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such particular valueand/or to the other particular value, along with all combinations withinsaid range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

1-15. (canceled)
 16. A process for the purification and separation of asynthesis gas stream containing hydrogen, carbon monoxide and carbondioxide, the method comprising the steps of: purifying the synthesis gasin a purification unit using a methanol washing step to remove carbondioxide to form a carbon dioxide depleted synthesis gas; purifying thecarbon dioxide depleted synthesis gas by adsorption to produce apurified synthesis gas stream; sending the purified synthesis gas streamto a cryogenic unit where the purified synthesis gas stream is cooledand separated by cryogenic separation in the cryogenic separation unitto produce an enriched carbon monoxide stream; and dividing the enrichedcarbon monoxide stream into a first part and a second part, wherein thefirst part is removed from the cryogenic separation unit to form a firststream, wherein the second part is sent to the purification unit whereinthe second part is warmed and then mixed with the first stream.
 17. Aprocess according to claim 16, wherein the second part of the enrichedcarbon monoxide stream is removed from the cryogenic separation unit ata temperature below 20° C.
 18. A process according to claim 17, whereinthe second part of the enriched carbon monoxide stream is removed fromthe cryogenic separation unit at a temperature above −65° C.
 19. Aprocess according to claim 16, wherein the second part of the enrichedcarbon monoxide stream is warmed in the purification unit to atemperature between −40° C. and 40° C. and then mixed with the firststream within the cryogenic separation unit.
 20. A process according toclaim 16, wherein the second part of the enriched carbon monoxide streamis warmed against the synthesis gas upstream of the methanol washcolumn.
 21. A process according to claim 16, wherein the second part ofthe enriched carbon monoxide stream is warmed against an intermediateliquid side-stream or bottom liquid stream from the methanol washcolumn.
 22. A process according to claim 16, wherein the second part ofthe enriched carbon monoxide stream is warmed against carbon dioxidedepleted synthesis gas from the methanol wash column.
 23. A processaccording to claim 16, wherein the second part of the enriched carbonmonoxide stream is warmed against a stream removed from a flash columnof the purification unit.
 24. An apparatus for the purification andseparation of a synthesis gas stream containing hydrogen, carbonmonoxide and carbon dioxide, the apparatus comprising: a purificationunit comprising a methanol washing column and a heat exchanger, thepurification unit configured to purify the synthesis gas to removecarbon dioxide; an adsorption unit in fluid communication with thepurification unit and configured to receive the synthesis gas from thepurification unit and provide further purification to produce a purifiedsynthesis gas stream; a cryogenic separation unit in fluid communicationwith the adsorption unit and configured to receive the purifiedsynthesis gas stream from the adsorption unit and cool and separate thepurified synthesis gas stream to form an enriched carbon monoxide streamand a hydrogen enriched stream, the cryogenic separation unit comprisinga column system and a heat exchange line; the heat exchange lineconfigured to warm the enriched carbon monoxide stream and the hydrogenenriched stream; means for dividing the enriched carbon monoxide streaminto a first stream and a second stream, wherein the heat exchanger ofthe purification unit is configured to receive the second stream fromthe cryogenic separation unit; and means for removing the second streamfrom the heat exchanger and mixing the second stream with the firststream.
 25. The apparatus according to claim 24, wherein the apparatusis configured such that the heat exchanger of the purification unitreceives the second stream at a temperature between −65° C. and 20° C.26. The apparatus according to claim 24, comprising a means for sendingto the heat exchanger synthesis gas upstream of the methanol wash columnof the purification unit.
 27. The apparatus according to claim 24,comprising a means for sending to the heat exchanger of the purificationunit an intermediate liquid side-stream from the methanol wash column.28. The apparatus according to claim 24, comprising a means for sendingto the heat exchanger of the purification unit a bottom liquid streamfrom the methanol wash column.
 29. The apparatus according to claim 24,comprising a means for sending to the heat exchanger of the purificationunit carbon dioxide depleted synthesis gas from the methanol washcolumn.
 30. The apparatus according to claim 24, wherein thepurification unit includes a flash column and means for sending liquidfrom the methanol wash column to the flash column and means for sendinga stream removed from the flash column to the heat exchanger of thepurification unit.